Tuesday, July 31, 2012

Finding the sand from the Elwha River

I've posted recently about the sand that is accreting to the beach near the river mouth of the Elwha River. I and others are tracking this carefully to determine if and when the removal of two dams on the river acts to halt or reverses chronic erosion along the delta. Last week I also went out for the first in a series of dive sessions, with other divers from the USGS, the Lower Elwha Klallam Tribe and the EPA, to monitor a series of sub-tidal sites near the Elwha River mouth (the red dots in the photo below):

At each site we are collecting a suite of data to describe the invertebrate and kelp community, as well as the substrate. We started this project in 2008, and more information on the project and the biological community off-shore of the Elwha can be found here.

The new sand on the beach made me think that we might see some new sand at some of our sites, especially the shallow sites to the east of the river mouth. But I was struck by how similar everything looked (as reported by the Peninsula Daily News), from a grain-size standpoint, to our surveys from the previous year. So we went hunting for it...

We decided to enter the water at our monitoring site "D1" and swim a transect to the river mouth along a line that is surveyed at least once a year by the USGS for bathymetry:

Preliminary results from the most recent USGS bathymetry survey, in May 2012, suggest that there had been new sediment added to this transect between September 2011 and May 2012:

This is a series of profiles collected along the transect since 2005 (courtesy of Andrew Stevens, USGS), showing the transition of this particular slice of the earth from a beach to a river mouth (between about 2005 and 2008), and the subsequent accumulation of sediment in the shallow coastal zone (since 2008). Between September 2011 and May 2012 there was more sediment added...and I wanted to know what its composition was...sand? sand and gravel? mud?

The video that we collected gives it away. I shot video at various depths as we moved up the slope towards the river mouth. Each of these video segments is prefaced by showing the profile again, with a black arrow that corresponds to our position on the profile. Enjoy!

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